1. Field of the Invention
This invention is directed to arc chute assemblies that assist in extinguishing arcs formed as the separable contacts of a circuit breaker open under load. More particularly, aspects of the invention are directed toward generation of increased gas during current interruption and directing the flow of those gases to promote arc cooling and more rapid termination of the arc, while at the same time, containing the increased gas pressure.
2. Background Information
The current interruption capability of air circuit breakers is dependent in part upon their ability to extinguish the arc that is generated when the breaker main contacts open. Even though the contacts separate, current continues to flow through the ionized gases formed by vaporization of the contacts and surrounding materials. Effective current limiting requires fast and efficient cooling of the arc. The arc is extinguished through transfer to a set of stacked metal plates in an arc chute. The basic geometry has been optimized over the years for the number of plates, plate spacing, and a variety of throat shapes. This stack of metal plates increases the arc voltage in an air circuit breaker to produce a current-limiting effect thereby providing downstream protection. The process of increased arc voltage results from cooling the arc and splitting the arc into series of arcs. Cooling results from arc attachment to the metal plates, vaporization of the plates and insulating materials, and discharge of the hot gases out of a vent. Arc splitting into a series of arcs also results in increased arc voltage due to additional cathode fall potentials. Magnetic materials, for example, steel, are used for the arc plates for their ability to attract the arc due to the self-induced magnetic field produced from the fault current. In addition, arc cooling depends on the gas flow over the plates (convection) and hot gas removal out of the vent of the circuit breaker. The volume of gas generated during current interruption has been enhanced by coating the support structure for the stacked metal plates with gas evolving materials such as cellulous filled melamine formaldehyde, glass polyester filled with alumina trihydrate (ATH) or by providing inserts made of such materials. While increasing the volume of gas generated during current interruption and enhancing its flow aids in extinguishing the arc, it also increases pressure within the circuit breaker, and therefore, on the arc chute and the circuit breaker casing. This can limit the current interruption capability of the circuit breaker.
There is a need, therefore, for improvements in arc chute assemblies for circuit breakers.